The border defines the boundary of the neighboring countries and is a natural flashpoint for international incidents. Over the past three hundred years or so, the Sino-Russian border, the world's longest, has been such a venue of frequent clashes. Figuring significantly among the archives of the Qing Foreign office and those held in trust by the National Palace Museum at Taipei for the Ministry of Foreign Affairs, are the many Qing territory treaties and treaty maps signed with Russia over the lands in the northeast, the north (Mongolia), and the northwest (Xinjiang). The sensitive, controversial nature of the documents had kept them sealed as "Strictly Confidential" for the longest time until 2001, when the Ministry entrusted the archives of historical documents to the Museum for safekeeping and digitization. Finally in 2007, they were declassified, and have since become open to public access, as well as for exhibition by the Museum. Hence the presentation of The Lost Frontier.

Xinjiang is situated in northwestern China, a pivotal hub for the Silk Road traffic and where the East meets the West. Since antiquity, peoples of diverse ethnicities have migrated to and through here, and mingled with one another. The Qianlong emperor (1736-1796) named the region Xinjiang, for New Territory. Before the lands became part of the Qing Empire, various nomadic tribes such as Kazakhs, Dzungars, and Kokand had called it home. After Qianlong put down Dzungars, three Tannu Uriankhai clans north of the Altai Mountains came to his rule successively. Soon the lands east to the Ayaguz River, southeast to Lake Balkash, as well as the area around Temurtu-nor (today's Lake Issyk-Kul, nor for lake) and the Narin River valley, were also brought into the Empire. However, the Empire started to decline during the reign of Daoguang (1821-1851) and went downhill from there. Its control over the vast yet remote Xinjiang was showing signs of weakness. Russia moved in eastwards opportunistically, setting up fortresses and seizing territories. After the 1860 Additional Treaty of Peking following the second Opium War, through a pretext of surveying the borders, the Czar pressured the Son of Heaven to cede the lands and acknowledge the Russia's de facto possession (on the grounds of being merely present where they were).

The Protocol of Chuguchak of 1864 (Sino-Russian joint survey of the Northwest) and three more "survey-conducting" sub-treaties, were signed during the reign of Tongzhi (1862-1875). Under Guangxu (1875-1909), more areas were signed over to Russia through the Treaty of Livadia, the Treaty of St. Petersburg and five subsequent sub-treaties. Thus in this way, by a series of re-demarcation treaties, the Empire's western frontiers gradually withdrew eastwards off the original demarcation line from Shabing dabakha (mountain pass) to the Pamir Mountains. All together, the statistics showed that between the two emperors of Tongzhi and Guangxu, a total of lands over 500,000 square kilometers were lost in the northwest.

The present exhibition is composed of three sections: first, "The Changing Borders," second, "Demarcating and Signposting: North, Middle, and South," and last, "In Conclusion." In addition to the secret treaties and maps previously archived as Confidential by the Ministry of Foreign Affairs, reports and charts submitted to His Majesty by the official delegations to the border negotiations are also exhibited.